Recently, non-contact IC tags have been now in high demand in many fields in addition to the physical distribution field, and thus even lower costs and higher performance are required. On the other hand, large-capacity memory cards are now in common use, and are broadly used in mobile digital devices, including digital cameras, mobile music players, and mobile information terminals. The further provision of a wireless communication function is expected to broaden the field of application of memory cards. In response to the expectation, an SD memory card with wireless interface function is disclosed in Japanese Patent Unexamined Publication No. 2001-195553 (hereinafter referred to as “Reference patent 1”).
The SD memory card disclosed in the above Reference patent 1 has a wireless controller separate from a functional unit as storage medium which is the main function. An antenna module with a loop antenna is connected to this wireless controller via an interface. The flash memory is a flash ROM which is a memory of the SD memory card, and it also stores a driver program for executing a wireless communication function. When an SD memory connected to this antenna module is installed in electronic equipment such as a mobile digital device, communications with external wireless communication equipment are performed via radio waves emitted from the loop antenna, utilizing the wireless communication function of the SD memory card, without any special operations.
However, since the antenna module is attached on the outside of one end of the SD memory card in the above structure, the entire size of the SD memory card becomes larger to accommodate the size of the loop antenna. Consequently, extra space for the loop antenna needs to be secured for installing the SD memory card into electronic equipment such as mobile digital devices, thus hindering downsizing.
Another idea disclosed is to build in the antenna along one end face of a side of the SD memory card where no connecting terminals are provided. This structure is applicable to communications using the 2.4-GHz band. However, for using the 13.56-MHz band, provision of sufficient length of antenna becomes problematic.
As a solution, a thin IC card in which electronic components are mounted on a flexible sheet holding an antenna coil is disclosed in Japanese Patent Unexamined Publication No. H11-134459 (hereinafter referred to as “Reference patent 2.”).
The thin IC card disclosed in the above Reference patent 2 is configured with a laminated body that consists of a flexible sheet of resin film holding an antenna coil. The resin film is divided in almost equal intervals in one direction. The laminated body is made by folding this resin film in units of these divisions and integrating them. Spiral conductive patterns, whose spiral centers match when folded and overlaid, are formed on at least one face of unit divisions of the resin film. In addition, the spiral conductive patterns in unit divisions are connected in series via predetermined connectors so that the electric current flows in the same winding direction when the spiral conductive patterns are folded and overlaid.
In cases when radio wave of a medium frequency is used, a remote ID tag configuring an antenna with a conductive pattern is disclosed in Japanese Patent Unexamined Publication No. H11-168406 (hereinafter referred to as “Reference patent 3”).
The antenna of the remote ID tag, disclosed in the above Reference patent 3, is formed by folding a flexible printed circuit board. In other words, multiple pattern coils provided on multiple flat sections of the flexible printed circuit board are folded such that two adjacent pattern coils with the flexible printed circuit board or insulating layer in between are wound in reverse. The antenna is configured by connecting one end of each of the pattern coils forming a single coil such that multiple pattern coils are wound in permutation in the same direction.
In this structure, the number of turns in one coil wound in permutation in the same direction can be increased to a predetermined number by increasing the number of foldings of the flexible printed circuit board. Accordingly, the antenna length can be easily extended to approximately 10 m. The antenna can thus be configured with a conductive pattern even if a medium frequency is used. Since the thin flexible printed circuit board is simply folded, the remote ID tag can be made thin.
In the invention disclosed in Reference patent 1, the entire size of the SD memory card becomes larger to accommodate the loop antenna, since the antenna module is attached externally at the end of the SD memory card. This hinders downsizing of mobile electronic devices.
In the inventions disclosed in Reference patents 2 and 3, the number of windings of the antenna is increased by folding the antenna pattern. However, no particular study of antenna characteristics has been carried out to investigate what happens when an LSI such as memory to be installed becomes larger in response to requirements for higher performance of IC card functions. In addition, variation in receiving sensitivity depending on the relative direction of the IC card toward a card reader has not been considered.